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Vintage Synth Repair

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Re: [vintagesynthrepair] Prophet rev 2

2009-09-09 by Stephen Jones

Sounds like the DAC is not getting some bits. I had the same problem with a P-600, scaling fine at the top but not at the bottom. The fault was that a chip had failed in front of the DAC and the DAC was not getting it's last few bits. There was a pair of 4000 series chips and one had failed partially, I will have to check the schematic later today to see what they were. I have a Rev. 2 to service tomorrow, I will take a look at the schemo today.

From: Terje Winther
To: vintagesynthrepair@yahoogroups.com
Sent: Wednesday, September 9, 2009 7:24:43 AM
Subject: [vintagesynthrepair] Prophet rev 2

Hi all,
I have a Prophet 5 rev 2 in for service. Originally the problem was
that autotune didn´t work.
Now autotune, program saving and retrieving as well as all voices
works great.
However, I am unable to get the keyboard to scale correctly. The top 3
octaves is correct, but the bottom two octaves are slightly off
scaling, enough to sound out of tune. I am able to trim the DAC
according to spec, and it helped, but not enough. I tried turning the
trimmer even further (lower than +5.000), and even though it helped,
it is not enough, and I reach the end of the trimmer. I checked the
power and it is slightly off (+15.3V and -14.75V). I could start total
recalibration of the whole synth, but that is a staggering 80+
trimmers (which are sealed now), and it seems a bit overkill when all
I need is adjustment of keyboard scaling.
Any hints appreciated.

Terje Winther
terje.winther@ wintherstormer. no
http://www.wintherstormer.no/


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